For prior art nonaqueous electrolyte batteries using light alkali metals such as lithium and sodium as the negative electrode active material, it was proposed to use metal oxides, halides, sulfides or the like as the positive electrode active material. As to primary batteries, those using manganese dioxide and carbon fluoride as the positive electrode active material have been commercially produced and marketed. As to secondary or rechargeable batteries, some lithium secondary batteries have also been commercially produced. These lithium secondary batteries used positive electrode active materials capable of intercalating and deintercalating lithium ions and exhibiting excellent cycle property, for example, titanium and molybdenum sulfides, vanadium oxides, and organic conductive polymers such as polyaniline while alloys of lithium with an alloying element such as aluminum were used as the negative electrode active material. The use of lithium alloys could significantly restrain the occurrence of shortcircuit due to dendrite growth as compared with a negative electrode formed solely of metallic lithium.
Under these circumstances, vanadium pentoxide is considered a promising positive electrode active material for nonaqueous electrolyte batteries in view of its relatively high voltage, chemical stability, and relatively good cycle performance. There is a strong desire to apply vanadium pentoxide to secondary batteries.
However, many problems are encountered by nonaqueous electrolyte batteries using vanadium pentoxide as the positive electrode active material. For example, vanadium pentoxide shows a characteristic undesirable discharge behavior that as charge/discharge cycles are repeated to and from 2 volts via flat regions of about 2.4 and about 3.3 volts at which it electro-chemically reacts with about 1 mol of lithium atom, its grain structure is gradually destroyed, resulting in a considerable loss of discharge capacity. One prior art attempt for overcoming such drawbacks of vanadium pentoxide is to quench molten vanadium pentoxide into amorphous form (see, for example, Solid State Ionics, 9 and 10, 649-657 (1983)). The use of amorphous vanadium pentoxide as positive electrodes is, however, generally accompanied by a lowering of discharge voltage and the advantageous feature of high voltage is lost.
Secondary batteries having a positive electrode of vanadium pentoxide might be fabricated by combining a lithium negative electrode with the vanadium pentoxide so as to take advantage of the first stage flat portion of more than about 2.5 volts of the latter. There would be obtained a secondary battery having a discharge capacity of about 150 mAH/g. The secondary batteries thus constructed, when subject to charge/discharge cycles between 3.8 V and 2.5 V, provide a capacity of 80 to 90% of the initial capacity after the second cycle of later and gradually lowers their discharge voltage. More particularly, the vanadium pentoxide positive electrode converts into a material represented by Li.sub.x V.sub.2 O.sub.5 during discharge by intercalating lithium ions interlaminarly. It can take up at most 1 mol of lithium ion per mol of V.sub.2 O.sub.5 at potentials of 3.8 to 2.5 V. During charging, the reaction of releasing lithium ions is induced. The lithium ions, once intercalated, cannot be deintercalated entirely from the vanadium pentoxide with about 10 to 20% of lithium ions left therein. This, combined with increased diffusion resistance within the vanadium pentoxide layer, results in a lowering of discharge voltage. Also the cycle performance is limited to about 100 cycles. For these drawbacks, the vanadium-lithium batteries are inferior in performance to the mainstream batteries as typified by nickel-cadmium batteries.
Regardless of its potential ability as a positive electrode active material as mentioned above, vanadium pentoxide has not been implemented as secondary batteries having high voltage and acceptable charge/discharge properties including cycle performance. There is a need for further research.
There is an increasing demand for secondary batteries while they are used in diversified environments. Under such situations, secondary batteries are quite often subject to overdischarge. Accordingly, recovery from overdischarge is one of most important features of secondary batteries. Where a battery is used as a memory backup power supply in computers, for example, discharge over its rated capacity is often imposed on the battery due to circuitry requirements and such a situation can continue for some time. If the battery cannot recover to its normal charge/discharge from such a heavy duty state, it is undesirably deficient for the memory backup use. Differently stated, the overdischarge property is one of important factors in selecting a battery for such use.
Nevertheless, secondary batteries using vanadium pentoxide as the positive electrode have not reached a satisfactory level with regard to the overcharge property. As described above, secondary batteries having a vanadium pentoxide positive electrode often use a negative electrode of metallic lithium or lithium alloy. For extended cycle life, the negative electrode generally has an electric capacity at least twice the electric capacity of a positive electrode. However, an increased electric capacity of the negative electrode allows the structural change of the positive electrode-forming vanadium pentoxide to proceed during overdischarge to such an irreversible extent that it is difficult to restore the capacity by re-charging.
More particularly, vanadium pentoxide has flat regions at 3.3 V and 2.4 V on discharge where substantial reaction takes place as previously described. It also has regions at 1.5 V and 1 V where considerable reaction takes place. The vanadium pentoxide that has experienced reaction past the regions of 1.5 V and 1 V has a grain structure completely different from the initial one so that a substantially reduced discharge capacity is available after re-charging. Optimum discharge property might be obtained by reducing the amount of metallic lithium used as the negative electrode or reducing the content of lithium in the lithium alloy used as the negative electrode, but at the expense of cycle performance. Then high current discharge is obtained no longer.
Not only the problem of discharge and cycle properties, but also the problem of overdischarge property are outstanding problems associated with vanadium pentoxide batteries. There is a desire for development of a nonaqueous electrolyte secondary battery using a vanadium pentoxide positive electrode by overcoming these ambivalent problems at the same time.
Therefore, an object of the present invention is to provide a nonaqueous electrolyte secondary battery of stable and reliable performance using vanadium pentoxide as the positive electrode active material, which battery has a high voltage output and improved charge/discharge cycle and overdischarge properties.